Abstract: The grading rings for valve tower in DC converter station are designed to avoid discharging in shielding system of DC converter valve for DC transmission. The structure model of shielding system is built by ANSYS software, and the surface electric field intensity for converter valve without and with grading rings is analyzed by using of Galerkin indirect boundary element method. Then the influences of height, position and distance between grading rings on surface electric field intensity of valve body structure are analyzed. By comparing design cases of different grading rings, the suitable scheme is concluded to meet constraint conditions. The surface electric field intensity of valve structure is reduced greatly with appropriate scheme and there is no corona on the grading ring either. The results show that the bigger the distance between grading rings around converter valve body structure is, the stronger the field intensity of midpoint on relevant edges is; the grading ring reduces the field intensity of relevant sharp corner for converter valve body structure greatly; the higher the grading ring is, the smaller the field intensity of sharp corner for converter valve body is.